linux-kernel - [PATCH] more ZERO_PAGE handling in follow

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Message-Id: <20080508185626.e14603d2.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 8 May 2008 18:56:26 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	nickpiggin@...oo.com.au, tonyb@...ernetics.com,
	mika.penttila@...umbus.fi
Subject: [PATCH] more ZERO_PAGE handling in follow_page() v2

updated against 2.6.26-rc1-git6.
Tested on a x86-64 server. No troubles in generated coredump file.
It seems to work well.

Thanks,
-Kame
==
follow_page() is called from get_user_pages(), which returns specified user page
in the kernel context. follow_page() can return 1) a page or 2) NULL or
3)ZERO_PAGE.

Now, follow_page() to unused pte returns NULL if page table exists. As a result
get_user_pages() calls handle_mm_fault() and allocate new memory.
This behavior increases memory consumption at coredump, which does
read-once-but-never-written page fault.
By returning ZERO_PAGE() against READ/ANON request, we can avoid it.

(Because exec's arguments copy needs to call handle_mm_fault at WRITE/ANON
 request, we just handle READ/ANON case here.)

Change log:
  - Rebased agasinst 2.6.26-rc1-git6
  - Rewrote patch description and Added comments.
  - fixed to check pte_present()/pte_none() in proper way.


Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

---
 mm/memory.c |   18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

Index: linux-2.6.26-rc1/mm/memory.c
===================================================================
--- linux-2.6.26-rc1.orig/mm/memory.c
+++ linux-2.6.26-rc1/mm/memory.c
@@ -962,15 +962,15 @@ struct page *follow_page(struct vm_area_
 	page = NULL;
 	pgd = pgd_offset(mm, address);
 	if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
-		goto no_page_table;
+		goto null_or_zeropage;
 
 	pud = pud_offset(pgd, address);
 	if (pud_none(*pud) || unlikely(pud_bad(*pud)))
-		goto no_page_table;
+		goto null_or_zeropage;
 	
 	pmd = pmd_offset(pud, address);
 	if (pmd_none(*pmd))
-		goto no_page_table;
+		goto null_or_zeropage;
 
 	if (pmd_huge(*pmd)) {
 		BUG_ON(flags & FOLL_GET);
@@ -979,15 +979,21 @@ struct page *follow_page(struct vm_area_
 	}
 
 	if (unlikely(pmd_bad(*pmd)))
-		goto no_page_table;
+		goto null_or_zeropage;
 
 	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
 	if (!ptep)
 		goto out;
 
 	pte = *ptep;
-	if (!pte_present(pte))
+	if (!pte_present(pte)) {
+		/* Read fault to empty pte can return ZERO_PAGE */
+		if (!(flags & FOLL_WRITE) && pte_none(pte)) {
+			pte_unmap_unlock(ptep, ptl);
+			goto null_or_zeropage;
+		}
 		goto unlock;
+	}
 	if ((flags & FOLL_WRITE) && !pte_write(pte))
 		goto unlock;
 	page = vm_normal_page(vma, address, pte);
@@ -1007,7 +1013,7 @@ unlock:
 out:
 	return page;
 
-no_page_table:
+null_or_zeropage:
 	/*
 	 * When core dumping an enormous anonymous area that nobody
 	 * has touched so far, we don't want to allocate page tables.

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